1. Field of the Invention
This invention relates generally to the control of paper positioning within the document handling and development systems of a xerographic system and, in particular, to the compensation for the positional errors of such paper.
2. Description of the Prior Art
It is a generally known technique to utilize servo mechanisms to provide controlled movement within positioning systems. For example, U.S. Pat. No. 4,641,070 to Pfizenmaier et. al. discloses a device for determining and adjusting the position of a printing press web. The device comprises two sensor units, a control unit responsive to output signals from the two sensors, and a device connected to the control unit for adjusting the position of the web.
U.S. Pat. No. 4,809,188 to Willits et. al. discloses a strip feeding control system for positioning and registering patterned material. A material moving system uses a drive roller and an associated pressure roller in a pinch roll system to move the material. A motion detecting sensing device and optical sensor provide outputs to a computer which provides control of material movement and provides signals for initiating machining of pattern material when the pattern is properly located relative to a designated work station. The material moving mechanism comprises a servo drive motor, an encoder, and material control rollers.
The prior art also discloses means for correction of errors in movement within coordinate positioning systems. For example, U.S. Pat. No. 4,581,566 to Ekstrom et al. discloses a system for automatically calibrating a robot in all degrees of freedom to compensate for inaccuracies in different coordinate directions. This system determines by means of sensing means the actual position reached by a measuring body, calculates the errors in different coordinate directions based on the the measured values, and compensates for the errors in respective coordinate directions.
Furthermore, it is a generally known technique to utilize servo controlled mechanisms to move and register paper within a xerographic system. U.S. Pat. No. 4,457,506 to Ashbee et. al. discloses a servo controlled automatic document feeder having a plurality of independently driven document transport devices. Each transport device is independently driven by a motor and a closed-loop servo. A common reference oscillator generates source signals for the servo loops. A micro computer operates on the source signals and generates reference signals for driving the transport devices. U.S. Pat. No. 4,816,867 discloses an electrophotographic copy machine comprising a document supporting glass plate and timing rollers for feeding copying paper to which the image on the photoreceptor drum is transferred. The document supporting glass plate defines a reference position with respect to the scanning direction, whereby the timing rollers are controllable such that the image present on the photoreceptor drum is transferred to a central position of the copying paper with respect to a copying paper feed direction.
A deficiency with prior art systems is the requirement that the the document feeding servomechanisms operate at a relatively slow rate of speed or in a bidirectional manner to enable proper positioning of the original document on the platen for accurate imaging. This can add to the complexity and inefficiency of the document feeder. Furthermore, the inability to correct or compensate for positional error in a xerographic system demands that relatively accurate servomechanisms be used for positioning the components of the xerographic system. It would, thus, be desirable to be able to rapidly position an original document and determine the positional error of such a document, with respect to a registration position, for correction by a copy sheet feeding subsystem.
It is an object of the present invention, therefore, to automatically compensate for paper and document misregistration due to system inertia, slippage and other factors by quantifying the degree of positional error. It is another object of the present invention to provide a new and improved servo-mechanical control system capable of determining the positioning error introduced while feeding an original document to an imaging location and counteracting the positional error using a secondary servomechanism.
Further advantages of the present invention will become apparent as the following description proceeds and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.